I have a 32GB iPhone 5s, am running the most current iOS (8.1.3)
None of the above steps actually accomplish PERMANENTLY DELETING OLD MESSAGES! I have over 8 GB of iMessage text messages. I have tried every suggested method of delete, from one at a time swiping, to "edit" , highlight and delete. It works in the moment, and LOOKS deleted, but doesn't actually REMOVE the messages. iMessage locks up often, then I am forced to reboot my phone. When I reboot, all messages back to 2013 are BACK ON MY PHONE!
I found a great app, and actually paid $35 to try to fix this! It allowed me to see and copy the old text and voice messages that I did want to keep. That's awesome, and I have saved to my desktop computer the files I want. Unfortunately this program doesn't have (apple) permission to delete the files.
Soooo......
I have set the "keep messages" function to only 30 days. There are STILL messages all the way back to early 2013.
I read that I needed to delete, then SYNC w/my iTunes to make them permanently go away. NOPE. THEY COME BACK.
I have done a complete restore, hoping it was just some bug, that would get cleaned out in the process. NOPE. THEY COME BACK.
In the meantime, functionality is HORRIBLE due to the fact that the app is over filled.
Someone please give me a real suggestion. PLEASE. I will be forever grateful.